Travelling-wave amplifying tube



Aug. 28, 1956 R. wARNEcKE ET A1. ,2,761,088

'TRAVELLING-WAVE AMPLIFYING TUBE Filed Feb. 16, 195o 4 sheetS-sheez 2 N Lf. e? s? Allg 28, 1956 R. wARNEcKE ET AL 2,761,088

TRAVELLING-WAVE AMPLIF'YING TUBE Filed Feb. 16. 1950 4 Sheets-Sheet 3 I MS Rum Mund.. we. www,...

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Allg. 28, 1956 R. WARNECKE ET AL 2,761,088

TRAVELLING-WAVE AMPLIFYING TUBE Filed Feb. 16. 1950 4 Sheets-Sheet 4 Pfg-4 Cu 'g1/Mns gi/mala Paris, France, as- Telegraphie Sans Ourinvention relates to an electronic tube which is particularly intended for power amplification in the `very short wave field, for example decimetric, centimetric or millimetric waves.` The electronic mechanism in this tube corresponds to a certainextent with that of travelling-wave tubes with a transverse magnetic tield, but as compared to those electronic tubes, the tube according toour invention has certain advantages which are ob` tained by means of a modification of the lprinciple of operation. As in the tubes hereinbefore mentioned, the operation of the novel tube is based on the interaction between a wave and an electron beam inside a timeconstant electric field and a time-constant magnetic field, the two iields being at right angles to the direction of propagation of the wave and of the electron beam. But whereas in the known tubes `a homogeneous magnetic field is applied, which is produced by a permanent oran electromagnet located outside the tube, in the: novel tube the'magneticv field is produced by a strong current flowing through anaxial conductor. This producesV a magnetic lield of cylindrical symmetry, which varies with the distance from the axis. This affects the shape of the electron trajectories and the operation of the tube described hereinafter. p

The invention will be more clearly understood by conslidering Figs. l, 3 and 4 which show a few non-limitative embodiments thereof in axial section. Fig.` 2 is ya eld distribution diagram, and` Fig. 5 shows a modification of construction of the cathode. j v

The principle of the novel tube is shown in Fig. 1. VIn this figure, 1 represents, the cathode in the shape of a broken ring, one end of which is connected to the axial conductor 2, and the other end to the current leadin wire 3. The heating voltagefor the annular cathode is applied across the lead-in wire 3 and the inner conductor 2 bymeans of the source Vf. The cathode is'located inside a groove in the axial conductor 2, which is insulated from' the other electrodes and is sealed in the glass walls 4 and 5 of thetube at the two ends 6 and `7 thereof; the conductor is of such a cross-section that a current of about 100 amperes or even more can liow along said con` ductor without heating the same to too high a temperature, i. e. without its temperature exceeding a value of a few hundreddegrees Centigrade. The conductor is surrounded by a helix S which acts :as a retardationline adapted toguide a wave in the direction of the axis at a substantially lower Velocity than that of light. The helix is supported `by a plurality of insulating rods 9. The wave to be ampliiied is introduced by means of a dipole 10 connected to one of the ends of the helix. The dipole 10 :at the input end is coupled to a waveguide 11 which is located outside the tube and supplies the control power. The piston 12 serves for matching the dipole 10 with the waveguide 11. The collection of-,the useful power at the output end is eiected in the same manner bymeans of the dipole 13 and the waveguide 14 which is matched by means of the piston 15. Beyond the output end is located a collector 16 adapted to collect the electrons' which have been emitted by the :annular cathode 1 .andhave passed through the electron .and wave inter'- action space between the helix and the axial conductor.

In order to produce a magnetic iield, a voltage V1 is applied between the ends of the axial conductor 2, said voltage causing a current of from a few tens to a few hundreds of amperes to flow through said conductor. The helix 8, which is' connected to the collector 16, is raised to a positive potential V2 of a few hundred volts or a few kv., with respect to the axial conductor 2 and to the cathode 1.

Starting from this illustration of the principle of the construction of the tube according to the invention, the principle of its operation will now be examined.

`The application of the voltage V2 produces a radial electric field Er between the axial conductor and the helix. The axialcurrent produces a magnetic field, the magnetic lines of force being circles about the axis, the magnetic induction B at a point P(r, z) being independent of the axial co-ordinate and being of the value Brera being `the distance between the point P' and the axis, J being the axial current, ,uo being the permeability of the vacuum. Since the electric force is in the radial direction 'and the magnetic iield is circular, the electrons emitted by the cathode are subjected to a force in the direction r, and if B is suliiciently high they do not reach the helix but move in the direction of z by following the trajectory 27 shown in Fig. l. This movement takes place in the same plane, i. e. there is no rotation of the electrons about the axis of the tube. The velocity vz of the electrons in the direction z can be readily calculated andthe following equation is obtained for 1a point located a distance r from the axis:

. fr Tft ro being the radius of the inner conductor. It can be seen that vz is dependent on r, that is to say that the velocity is not constant along the trajectory. In order to obtain an interaction between the electrons :and `the wave inside the electric field of said wave, it is necessary for the mean value of vz to be equal to the velocity of propagation of the wave with reference to the direction z. Take a numerical example: r0=0.5 cm., radius of the helix r1t=2 cm. and ve=l.5 l09 cm./s. that corresponds to a twentieth of the velocity of light for r=1.25 cm., that is to say to the middle of the space between the inner conductor and the helix. With .9 2 15 -slL m 1.76X10 andy@ 1.26X10 Aem T loge E it is found that the necessary current l is about 500 amperes. This value is rather high but it is possible to construct the inner conductor of a material of very good conductivity, of copper for example, in such a manner that the power applied to said conductor still remains fairly low, about a few tens of watts for example.

The foregoing considerations only relate to the static behaviour of the tube and are intended to show in particular the existence of a velocity component of the elec-Y trons in the direction z of the axis. The following explanations will make it possible to understand the dynamic behaviour of the tube as an amplifier of a wave guided by a retardation line along the direction of the axis of the tube at .a velocity of propagation which is equal to' the mean velocity of theelectrons in the same direction.

In the space between the inner conductor and the helix, the electric vector of the guided wave bas radial and longitudinal components. The action of the radial vector (Fig. 2) will first be examined, taking into account the fact that if an electron inside a magnetic field is subjected to the force of an electric field, the electron obtains an acceleration in a direction at right angles to the directions of the electric field and the magnetic field. In Fig. 2, 2 is the inner conductor, 8 the sections of the convolutions of the helix, the lines shown representing the lines of force of the electric field of the wave. An electron which is moving in the direction z and which is located at the point A in an electric field that retards the movement directed towards the helix, is slowed down if the magnetic field at right angles to the plane of the drawing is directed from the observer towards the paper (the current flowing through the axial conductor is in that case directed from right to left of the figure). An electron which is located at the point B is accelerated. There is therefore a grouping of the electrons adjacent the point C in `an electric field which, without a magnetic field, would have retarded the electrons. Now, in a magnetic field, the retardation is replaced by a lateral movement; the electrons retain their velocity but move towards the helix and, after a certain travel, reach the helix with a substantially smaller energy than that which corresponds to the potential of the helix. The difference of energy is converted into electromagnetic energy, in other words, it has been used for the amplification of the wave along the path z of same and of the electrons. Since the power of the wave is higher at the output end than at the input end, the tube can'be used as a power amplier.

The tube according to Fig. 1 only serves as a nonlimitative example for describing the principle of the invention. Figs. 3, 4 and 5 show several modifications of construction of the tube according to the invention.

In Fig. 3, another shape for the retardation line has been shown. The line also has `an inner conductor 2 through which flows a current that produces a magnetic field. But instead of a helix, use is made as an outer conductor of a metal cylinder 17 which is provided with numerous annular diaphragms 13, the inner conductor passing through the holes in said diaphragms. Itis easily possible to choose the dimensions of the cylinder and of the diaphragms in such a manner that a very substantial retardation of the wave is obtained with .respect to the velocity of light. The other reference numerals of Fig. 3 have the same meaning as those of Fig. 1.

The coupling of the input generator and the collecting of the amplified power are effected by means of coupling loops 19, 20 respectively. As compared with the helicalshaped retardation line, this shape of the line has the disadvantage that, with the dimensions given, the tube can only be used in a restricted range of frequencies; on the other hand, it has the advantage that the dissipated duces the magnetic eld necessary for the operation of the-tube. It is -obvious that this construction, as compared with the constructions of Figs. 1 and 3, has the advantage of avoiding the necessity of providing glass-tometal seals through which a current of fairly high value has to pass, since such seals, in the technique of electronic tubes, always involve certain difficulties. The other reference numerals have the same meaning as in the other figures.

In Figs. l and 3, the cathode is located adjacent the axial conductor which is placed inside the vacuum chamber. In Fig. 5, a modification is shown in which the emitting surface of the cathode directly forms a portion of the inner conductor. Only this conductor 2 is shown, it being understood that'the construction applies to Figs. l and 3 without making any other modification in same. The portions of this conductor where the surface is not covered with an emitting layer are made of a metal of good conductivity, normally copper. At the places where it is desired to have an emitting surface on the cylindrical axial conductor, the resistance of the conductor is increased. This is effected by replacing the good conducting metal by another metal, nickel for example, and by making `the'conductor hollow. In Fig. 5 this method has been shown diagrammatically; at the locations 23 a portion of the conductor 2 has been replaced by a nickel annulus, the A outer surface of which is covered with a layerof barium oxide in order to obtain a heavy electron current. Contrary to the previously described embodiments, the single cathode has been replaced by a. plurality of cathodes spaced along the axis of the tube, this modification having been found to favor the opera-I tion of the tube.

-The construction according to Fig. 5 has the advantage thatit is possible to use the current required for obtaining the magneticA field also as the heating current for the cathodes. .On the other hand, its disadvantage consists in the fact that it is only possible to vary the magnetic field very slightly by varying the axial current, owing to power and the useful power which it can supply are substantially higher than in the case of the helix.

` In Fig. 4, a modification is shown which is characterised by the fact that the conductor that produces the magnetic field is located outside the vacuum chamber of the tube. As in the construction of Fig. l, the retardation of the wave is obtained by means of a helix 8 which is supported by the wall of a glass tube 21 and which, by way of example, may be partly embedded in the inner wall of said tube. Instead of being provided with a solid thee variation of the temperature of the cathode which is only permissible within narrow limits.

The invention is not restricted to the examples shown in Figs. 1, 3, 4 :and 5 which are only intended to show the principle. It is possible for example to alter the manner of -coupling the generator and the load to the retardation line by using couplings of any nature, either inductive, 'capacitative or mixed, fas i's at present well known in highfrequency technique. The annuar cathode may be replaced by a cathode of any other shape. The invention generally consists in a tube in which an electron vbeam movesinside a transverse electric field and a magnetic field of cylindrical symmetry, said magnetic eld being produced by a current owing through an axial conductor.

whatwe claim is:

l.' A11 electron discharge tube comprising a vacuumtight envelope, an electron-emissive source, means for concentrating the electrons emitted by the source into a beam, a delay line of hollow cylindrical structure having input and output extremities and defining an electron and axial conductor, a hollow tube 22 passes axially through the container, the walls of said tube being vacuum-tight so that the inside thereof is outside the vacuum. This tube -22 may be constructed, as shown in the figure, of conducting material and in this case the heating source Vf is connected between said tube 22 and the cathode lead-in wire 3. But the tube 22 may also be made of insulating material, and in this case it is necessary to provide asecond cathode lead-in wire and to connect the heating source across the two lead-in wires. The solid conductor 2 is inserted into said tube 22, and through said-conductor 2 is made to ow the current which prowlave interaction space, means for causing the beam to pass laxially through the interaction space, wave input means coupled to the input extremity of the delay line, the delay characteristic of the line determining the phase propagation velocity of the wave in the line, terminal connections for applying a potential to the delay line thereby to produce a transverse time-constant electric field in the interaction space, and means for producing in the interaction space a time-constant magnetic field of cylindrical symmetry with circular lines of force :about the axis of the delay line, the strength of the magnetic field being predetermined thereby substantially to obtain equality between the 'average axial velocity of the beam in the interaction space and the wave phase propagation velocity.

2. Tube 'according to claim l, wherein said means for producing la time-constant magnetic field comprises a straight conductor extending along the 'axis of the delay line, and means for passing a direct current through the conductor thereby to produce the magnetic field of cylindrcal symmetry.

3. Tube according to claim 2, wherein the stnaight con ductor is located inside the envelope and extends through insulating elements at the ends thereof, the conductor having extremities extending outside of the envelope, 'and terminals for connecting a direct current source between said extremities.

4. Tube according to claim 3, wherein the electronemissive sou-ree comprises atleast one cathode located on the straight conductor near the input extremity of the delay line.

5. Tube according `to claim 4, wherein means are provided in the straight conductor structure for increasing the resistance thereof at spaced points.

6. Tube according to cllaim 5, wherein said means in the straight conductor structure comprise cylinders of a material of higher resistance than the remainder of the conductor :and located in the body thereof at said spaced points.

7. Tube according to claim 5, wherein said means in the straight conductor structure comprise cylinders Whose cross-sectional area is decreased by providing an axial passage therein and which arelocated in the body of the straight conductor at said spaced points.

8. Tube according to claim 2, wherein the envelope inclu-des a hollow tubel extending @along the inside of the cylinder of the delay line, the straight conductor extending through the tube and being located outside the envelope.

9. Tube according to claim 2, wherein the electron emissive source comprises an :annulus connected to the` Y 35 10. An electron discharge tube comprising a vacuumstraight axial conductor.

` concentrating the electrons emitted by the source tight enclosure, an electron-emissive source, means for into a hollow annular beam, a delay line of hollow cylindrical structure having input and output extremities and defining an electron and Wave interaction space, means for causing the beam to pass through the interaction space, wave input means coupled to the input extremity of thedelay line, the delay characteristic of the line determining the phase propagation velocity of the Wave in the line, terminal Iconnections for applying a potential to` the delay line thereby to produce a transverse time-constant electric field in the interaction space, 'a straight conductor extending along the axis of the delay line inside the annular space occupied by the beam, and means for passing a direct current through the conductor thereby to produce :about the axis of the delay line a time-constant magnetic iiel'd with circular lines of force of desired intensity to predetermine substantial equality between the average axial velocity of the beam and the Wave phase propagation velocity.

References Cited in the file of this patent UNITED STATES PATENTS 2,211,859 Percival Aug. 20, 1940 2,367,295 Llewellyn Jan. 16, 1945 2,439,401 Smith Apr. 13, 1948 2,511,407 Kleen et al. June 13, 1950 2,531,972 Doehler et al Nov. 28, 1950 2,559,581 Bailey July 10, 1951 2,566,087 Lerbs Aug. 28, 1951 2,607,904 Lerbs Aug. 19, 1952 2,610,308 Touraton et al. Sept. 9, 1952 OTHER REFERENCES Article by Borssart & Doehler, pp. 328-338, Annales de R'adioelectricite, vol. 3, No. 14, October 1948. 

